fix: Get(nil) fast-path and return correct first offset#18093

phrwlk wants to merge 2 commits intoerigontech:mainerigontech/erigon:mainfrom

phrwlk:fix/bptree-get-nil-first-offsetphrwlk/erigon:fix/bptree-get-nil-first-offsetCopy head branch name to clipboard